In one existing type of General Packet Radio System (GPRS), that uses only Gaussian Mean Shift Keying (GMSK) modulation, the transmitted power of a dual timeslot transmitter is changed directly from the power level of the first timeslot to the level of the second timeslot (i.e., there is no power ramping between adjacent timeslots).
However, in at least one proposed dual timeslot system known as Enhanced General Packet Radio Service (EGPRS), both GMSK and 8-PSK modulation is used, and the modulation sequence is changed from GMSK to 8-PSK, or vice versa, during the transition period. However, this approach would thus require that both timeslots are driven in the EDGE (Enhanced Data rate for Global Evolution) Mode, characterized by having fixed power amplifier (PA) gain power control, if at least one of the transmitted timeslots contains 8-PSK modulation. If only GMSK modulation is used in both timeslots, the transmitter would operate in the GMSK mode with a variable gain PA.
This approach would also set Iref according to the timeslot having the higher power level, and would drive the PA during both timeslots with a common value of Iref (Iref sets the PA quiescent current in the EDGE mode).
The inventors have realized that several problems can be experienced using the approach outlined above. A first problem relates to the available PA output power with the PA operated in the EDGE Mode with GMSK modulation. The GMSK power in the EDGE Mode is currently not defined in the PA specifications, and to operate the PA in the EDGE mode, with both GMSK and 8PSK modulation, would almost double the number of PA parameters that would need to be specified. With current commercially available PAs, that operate in the 850/900 MHz band, the power class drop to +29 dBm is a requirement, but to achieve this the transmit (TX) gain digital attenuator would need to be set to almost its minimum value to obtain the specification requirement of +26.5 dBm output power, in extreme conditions, with a safe margin. This TX gain attenuator setting difference also results in an additional tuning operation during manufacturing. In the 1800/1900 MHz band there is also a problem achieving +30 dBm power class (tolerance +/2, +/−2.5 in extremes), even with minimum TX gain attenuation. This may imply that the only possible solution in the 1800/1900 MHz band (GMSK) to reduce power is a class drop to +24 dBm.
A second problem relates to the production tuning operations, as they would be made more complicated if the same Iref value is used for both timeslots. For example, with the maximum Iref value all TX power levels need to be tuned, with the middle Iref value two thirds of the power levels would have to be re-tuned, and one third of the power levels would be required to be tuned three times. In addition, all of the required tuning tables would need to be saved in the software of the mobile station.
A third problem relates to avoiding discontinuities in the TX Inphase and Quadrature phase (I/Q) signals to ensure a smooth power transition between the 8-PSK and GMSK (and vice versa) time slots.
A fourth problem arises from the use of the same value of Iref for both timeslots, as this causes poor efficiency in the timeslot having the lower TX power level.
A fifth problem arises from the power detector sensitivity of the at least some commercially available PAs. The PA power detector in essence loses sensitivity with a maximum Iref at +19 dBm, which means that the operation of the power control loop is impaired with power levels below that value.